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<TITLE>Custom error responses</TITLE>
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<H1>Custom error responses</H1>

<DL>

<DT>Purpose

  <DD>Additional functionality. Allows webmasters to configure the response of
      Apache to some error or problem.

      <P>Customizable responses can be defined to be activated in the
      event of a server detected error or problem.

      <P>e.g. if a script crashes and produces a "500 Server Error"
      response, then this response can be replaced with either some
      friendlier text or by a redirection to another URL (local or
      external).
	
      <P>
	
<DT>Old behavior

  <DD>NCSA httpd 1.3 would return some boring old error/problem message 
      which would often be meaningless to the user, and would provide no 
      means of logging the symptoms which caused it.<BR>

      <P>

<DT>New behavior

  <DD>The server can be asked to;
  <OL>
    <LI>Display some other text, instead of the NCSA hard coded messages, or
    <LI>redirect to a local URL, or
    <LI>redirect to an external URL.
  </OL>

  <P>Redirecting to another URL can be useful, but only if some information
     can be passed which can then be used to explain and/or log the error/problem
     more clearly.

  <P>To achieve this, Apache will define new CGI-like environment
     variables, e.g.

  <blockquote><code>
REDIRECT_HTTP_ACCEPT=*/*, image/gif, image/x-xbitmap, image/jpeg <br>
REDIRECT_HTTP_USER_AGENT=Mozilla/1.1b2 (X11; I; HP-UX A.09.05 9000/712) <br>
REDIRECT_PATH=.:/bin:/usr/local/bin:/etc <br>
REDIRECT_QUERY_STRING= <br>
REDIRECT_REMOTE_ADDR=121.345.78.123 <br>
REDIRECT_REMOTE_HOST=ooh.ahhh.com <br>
REDIRECT_SERVER_NAME=crash.bang.edu <br>
REDIRECT_SERVER_PORT=80 <br>
REDIRECT_SERVER_SOFTWARE=Apache/0.8.15 <br>
REDIRECT_URL=/cgi-bin/buggy.pl <br>
  </code></blockquote>

  <P>note the <code>REDIRECT_</code> prefix. 

  <P>At least <code>REDIRECT_URL</code> and <code>REDIRECT_QUERY_STRING</code> will
     be passed to the new URL (assuming it's a cgi-script or a cgi-include). The
     other variables will exist only if they existed prior to the error/problem.
     <b>None</b> of these will be set if your ErrorDocument is an
     <i>external</i> redirect (i.e. anything starting with a protocol name
     like <code>http:</code>, even if it refers to the same host as the
     server).<p>

<DT>Configuration

  <DD> Use of "ErrorDocument" is enabled for .htaccess files when the
       <A HREF="mod/core.html#allowoverride">"FileInfo" override</A> is allowed.

  <P>Here are some examples...

  <blockquote><code>
ErrorDocument 500 /cgi-bin/crash-recover <br>
ErrorDocument 500 "Sorry, our script crashed. Oh dear<br>
ErrorDocument 500 http://xxx/ <br>
ErrorDocument 404 /Lame_excuses/not_found.html  <br>
ErrorDocument 401 /Subscription/how_to_subscribe.html
  </code></blockquote>

  <P>The syntax is, 

  <P><code><A HREF="mod/core.html#errordocument">ErrorDocument</A></code>
&lt;3-digit-code&gt; action 

  <P>where the action can be,

  <OL>
    <LI>Text to be displayed.  Prefix the text with a quote (&quot;). Whatever
        follows the quote is displayed. <em>Note: the (&quot;) prefix isn't 
        displayed.</em>

    <LI>An external URL to redirect to.

    <LI>A local URL to redirect to.

  </OL>
</DL>

<P><HR><P>

<h2>Custom error responses and redirects</H2>

<DL>

<DT>Purpose

  <DD>Apache's behavior to redirected URLs has been modified so that additional
      environment variables are available to a script/server-include.<p>

<DT>Old behavior

  <DD>Standard CGI vars were made available to a script which has been
      redirected to. No indication of where the redirection came from was provided.

  <p>

<DT>New behavior
  <DD>

A new batch of environment variables will be initialized for use by a
script which has been redirected to.  Each new variable will have the
prefix <code>REDIRECT_</code>.  <code>REDIRECT_</code> environment
variables are created from the CGI environment variables which existed
prior to the redirect, they are renamed with a <code>REDIRECT_</code>
prefix, i.e. <code>HTTP_USER_AGENT</code> becomes
<code>REDIRECT_HTTP_USER_AGENT</code>.  In addition to these new
variables, Apache will define <code>REDIRECT_URL</code> and
<code>REDIRECT_STATUS</code> to help the script trace its origin.
Both the original URL and the URL being redirected to can be logged in
the access log.

</DL>

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